1. Field of the Invention
The present invention relates to an apparatus and a cutting unit for comminuting large pieces of cuttable material
2. Description of the Background Art
The purpose of industrial comminuting of material is oftentimes the fabrication of an intermediate product of defined shape and size, which is subsequently used as source material for the production steps of another production process. The production of granulate is an example, which is further processed in extrusion devices. The uniformity in shape and size are thereby the determining factors for the quality of the intermediate product.
The operators of comminuting devices are regularly presented with a challenge, when large pieces of material, for example, bales of rubber, are to be reduced to a relatively small end size, for example, to granules. Generally, multi-step comminuting is used for applications of this kind, whereby several cutting mills of varying size, one behind the other, are provided. The end product of the previous cutting mill thereby serves as the source material for the next cutting mill. In this way, a step-by-step comminuting of the material to the desired end size takes place.
The advantage of this method is that it yields high-quality granulate material. The disadvantages, however, are economical because several cutting mills have to be on hand. Due to the need and operation of several comminuting machines, there are, apart from the expenditures for their acquisition, maintenance, and operation, additional costs for the required surge tank capacities, means of transportation, and the additional space requirement.
To circumvent these disadvantages, devices have been made for special applications, for example, the comminuting of bales of rubber, which make it possible to comminute inside a housing the original material to the desired fineness of the end product. Devices of this kind have a rotating, cylinder-shaped rotor provided with axially oriented knives, which are evenly distributed around its periphery moving in a mutual blade flight circle. For the purpose of comminuting, the knives interact with stator knives, which are tangent to the blade flight circle. A perforated strainer is arranged over a partial area of the blade flight circle, via which the sufficiently comminuted material is discharged. During comminuting, the material remains in the cutting area of the knife until the particles are small enough to pass through the perforations of the strainer. In other words, in devices of that kind, the pre and post comminuting is done simultaneously with one piece of equipment with the same comminuting tools.
One of the advantages thereof is the need for only one comminuting device, which in contrast with the previously described multi-step variation is a cost and space-saving way of comminuting. However, the consequence of the one-step comminuting is that the entire comminuting process, that is, coarse and fine comminuting, is done by the same comminuting tools. However, since the individual comminuting steps have different objectives due to different initial conditions, it would be desirable in view of quality improvement to adjust the respective comminuting tools to the specific requirements of each comminuting step by using particularly suitable comminuting tools. This would not be possible with one-step comminuting, which is the reason why, from a technical viewpoint, one-step comminuting is a compromise.
An additional factor in the comminuting of bales of rubber or extruded synthetic hollow sections is that for the initial cutting of the material, very high forces must be applied to make comminuting possible. During that process, high impact forces are applied to the material by the comminuting tools. These cause extremely high mechanical stress, which must be taken into consideration in regard to the dimensional design of the device. Furthermore, this way of comminuting characteristically generates considerable noise, which makes it necessary to encapsulate the device to protect the personnel.
Additionally, the resistance encountered at the initial cutting of the material by the comminuting tools causes a reduction of the rotational speed of the comminuting rotor. To offset this loss of rotational speed, a strong short-term power surge to the drive motor occurs. Since energy costs are calculated not by the average energy consumption but by peak demand, this leads to a superproportional increase in energy costs.